A hair styling appliance is a thermal device for styling hair. A hair styling appliance styles hair by heating the hair above a transition temperature where it becomes mouldable. Depending on the type, thickness, condition and quantity of hair, the transition temperature may be a temperature in the range of approximately 160° C.-200° C.
A hair styling appliance can be employed to straighten, curl and/or crimp hair.
A hair styling appliance for straightening hair is commonly referred to as a “straightening iron” or “hair straightener”. FIG. 1 depicts an example of a typical hair straightener (1). The hair straightener (1) includes first and second jaws (2a, 2b). Each jaw comprises a heater that includes a heating element (not shown) arranged in thermal contact with a heatable plate (3a, 3b). The heatable plates are substantially flat and are arranged on the inside surfaces of the jaws in an opposing formation. During the straightening process, the hair is clamped between the hot heatable plates and then pulled under tension through the plates so as to mould it into a straightened form. The hair straightener may also be used to curl hair by rotating the hair straightener 180° towards the head prior to pulling the hair through the hot heatable plates.
Hair styling appliances for curling hair include “curling tongs” and “curling wands”. FIG. 2 depicts an example of a typical curling tong (1′). The curling tong includes first and second jaws (2a′, 2b′). The first jaw comprises a heater having a cylindrical or rod-like form. The heater includes a heating element arranged in thermal contact with a substantially cylindrical heatable plate (3′). The second jaw comprises a clamp portion (4′) with a concave cylindrical clamp face that is shaped to conform to the cylindrical heatable plate. During the curling process, the hair is wound around the hot cylindrical heatable plate (3′) and clamped by the clamp portion (4′) until it is moulded into a curled form.
A hair styling appliance for crimping hair is commonly referred to as a “crimping iron”. FIG. 3 depicts an example of a typical crimping iron (1″). The crimping iron includes first and second jaws (2a″, 2b″). Each jaw comprises a heater. Each heater includes a heating element arranged in thermal contact with heatable plate (3a″, 3b″). The heating plates have a saw tooth (corrugated, ribbed) configuration surface and are arranged on the inside surfaces of the jaws in an opposing formation. During the crimping process, the hair is clamped between the hot heatable plates until it is moulded into a crimped shape.
FIG. 4 schematically depicts an internal arrangement (10) of a typical hair styling appliance. This particular internal arrangement relates to a hair straightener having a pair of heaters (11a, 11b) as depicted in FIG. 1. The hair styling appliance includes a control PCB (12) having voltage detection means (13) and thermal control means (14). The voltage detection means is provided to control the input voltage from the power supply (15). The thermal control means is provided to control the operation of the heaters. One or more temperature sensors (16) are mounted in association with the heaters so as to provide feedback control data to the thermal control means. A user interface (17) is provided to allow a user to control the operation of the hair appliance as required.
Conventional hair styling appliances are typically characterised by a lack of thermal control. The lack of thermal control can restrict the styling performance of a hair styling appliance and/or may cause damage to the hair. For example, a hair styling appliance with limited thermal control may provide a fluctuating, uneven, excessive and/or insufficient heating effect. The hair styling appliance may provide an uncontrollable heating effect whereby the temperature of a heating plate fluctuates during the styling process. The hair styling appliance may provide an undesirable heating effect whereby the temperature varies along the length of a heater. The hair styling appliance may provide an excessive heating effect whereby a heatable plate becomes hot enough to damage hair, particularly “virgin” hair on top of the head. The hair styling appliance may provide an insufficient heating effect whereby a heatable plate does not become or remain hot enough to heat the hair to the transition temperature. This may result in repeated use of the hair styling appliance which can cause damage and cuticle stripping.
The thermal control may be compromised if the hair styling appliance has a long thermal time constant. The thermal time constant may be unduly long if a heatable plate has poor thermal conductivity and/or a large thermal mass. The long thermal time constant may cause the temperature of the heatable plate to fluctuate during the styling process due to a time lag between the dissipation of heat from the heatable plate to the hair and supply of heat from a heating element to the heatable plate. This thermal control problem is exacerbated if the hair styling appliance is used to style thicker, wetter and/or greasier hair. Thicker, wetter and/or greasier hair has a larger heat mass than average hair and it so requires more heat energy to be delivered to the hair during the styling process. Accordingly, the temperature of the heatable plate is likely to drop below the transition temperature whilst styling these types of hair and so the performance of the hair styling appliance is compromised. Previously, this thermal control problem has been addressed by using a higher starting temperature so as to try and maintain the temperature of the heatable plate above the transition temperature. However, it has been found that this higher starting temperature is likely to cause damage to the hair and so it is an unsuitable solution.
The thermal control of a hair styling appliance may be compromised by the position of the temperature sensor. In normal use, it is rare for hair to be evenly loaded along the length of the heatable plate. Indeed, hair is typically loaded at one end of the heatable plate. If the temperature sensor is arranged in association with the unloaded region of the heatable plate, then it will erroneously determine the heatable plate is at the desired operating temperature, even though the loaded region of the heatable plate is cooling as it dissipates heat to the hair. Hence, a temperature gradient will form along the length of the heatable plate and the hair styling appliance will not provide a sufficient heating effect on the hair. Alternatively, if the temperature sensor is arranged in association with the loaded region of the heatable plate, it will detect the cooling of the loaded region. The heating element will then be activated to provide further heating of the heatable plate and thereby maintain the loaded region of the heatable plate at the desired operating temperature. Since the unloaded region has not dissipated any heat to the hair, the further heating will create a temperature gradient along the length of the heatable plate. Moreover, the further heating of the heatable plate can result in the temperature of the unloaded region becoming hot enough to cause damage to any hair that strays into the unloaded region.
FIG. 5 depicts a schematic exploded view of an example of a conventional heater so as to illustrate the effect of uneven hair distribution. The heater (20) includes a heating element (21), a substantially flat heatable plate (22) and a temperature sensor (23) positioned between the heatable plate and the heating element. The heating element is arranged in thermal contact with the heatable plate so as to heat the plate during use. The temperature sensor is positioned towards the first end (22a) of the heatable plate. Hence, the temperature sensor is able to detect the temperature of the first end region of the heatable plate. In accordance with normal usage, the hair (24) is unevenly loaded in the hair styling appliance and is positioned close to the second end (22b) of the heatable plate. Hence, the second end region of the heatable plate is arranged in thermal contact with the hair so as to heat the hair. Since the temperature sensor is remote from the hair, the temperature sensor does not detect the cooling of the second end region of the heatable plate as it dissipates heat to the hair. Accordingly, a temperature gradient is created along the length of the heating plate as the second end region of the heating plate becomes cooler than the first end region of the heating plate.